Gragas and Poppy have different strengths than Illaoi and Graves.
Gragas was OP before his barrels were changed to deal reduced damage to minions because he offered tankiness, siege/waveclear, disengage and engage. He was good (and better than a lot of champions) at almost all aspects of the game at almost all points in the game. This caused him to crowd out almost all other picks.
Of course, I am exaggerating here, but you get the point.
Illaoi offers limited power to her team. As she can only really get engaged on, if she is behind, she is much weaker than almost any other champion in her position. Riot may have decided that her fallback pattern could be stalling a side wave. This is a good fallback pattern as when she is ahead it isn't too useful - her tools are much much weaker in offensive sieges.
Basically, they are different champions. They need different limitations. Their abilities need to do different things.